At present, a refrigeration technology in a room temperature region which closely relates to a human daily life, for example, a refrigerator, a freezer, a room air conditioner, and the like, mainly employs a gas compression/expansion cycle. However, a serious problem of environmental destruction is caused by specific freon gases discharged into the environment as to the refrigeration technology based on the gas compression/expansion cycle. Further, CFC substitutes also have a problem of an adverse affect to the environment. To cope with the above problems, the refrigeration technology is recently improved using a natural refrigerant (CO2 and the like) and isobutane. It is required from the above background to practically use a refrigeration technology which does not cause the problem of environmental destruction due to discharge of working gases, is clean, and has high efficiency.
Recently, an expectation is increased to a magnetic refrigeration technology as one of the refrigeration technologies which take environment into consideration and have high efficiency. And a magnetic refrigeration technology in a room temperature region is actively researched and developed. A magnetocaloric effect in iron (Fe) was discovered. The magnetocaloric effect is such a phenomenon that when an external magnetic field applied to a magnetic material is changed in an adiabatic state, the temperature of the magnetic material is changed.
In magnetic refrigeration, a low temperature is created making use of the magnetocaloric effect as described below. In a magnetic material, entropy is changed between a state in which a magnetic field is applied thereto and a state in which the magnetic field is removed therefrom due to the difference between the degrees of freedom of an electromagnetic spin system. When the entropy is changed, the entropy migrates between the electron spin system and a lattice system. The magnetic refrigeration uses a magnetic material having a large electron spin. A low temperature is created by transmitting entropy between the electron spin system and the lattice system making use of a large amount of change of the entropy between when the magnetic field is applied and when the magnetic field is removed.
A refrigeration device for a magnetic refrigeration system was developed using paramagnetic salts such as Gd2(SO4)3-8H2O and the like and paramagnetic compounds represented by Gd3Ga5O12 (gadolinium gallium garnet; GGG) acting as magnetic refrigeration materials having a magnetocaloric effect. In the refrigeration device for realizing the magnetic refrigeration using paramagnetic materials is mainly applied to low temperature region of 20 K or less, and a magnetic field of about 10 tesla, which can be obtained using a superconducting magnet, is used.
In contrast, to realize magnetic refrigeration at a higher temperature, magnetic refrigeration making use of magnetic phase transition between a paramagnetic state and a ferromagnetic state of a ferromagnetic material has been actively studied. There are proposed many magnetic materials including rare earths having a large electron spin per unit volume such as rare earth element simple substances of a lanthanoide rare earth elements such as Pr, Nd, Dy, Er, Tm, Gd, and the like, at least two kinds of rare earth alloy materials such as Gd—Y, Gd—Dy, rare earth intermetallic compounds such as RAl2(R represents rare earth elements, which is the same in the following description), RNi2, GdPd, and the like.
Magnetic refrigeration in a room temperature region was realized using a ferromagnetic material Gd having a ferromagnetic phase transition temperature (Tc) of about 294 K. However, it was difficult to achieve a steady state although a refrigerating cycle was continuously operated. There was an attempt to use the lattice entropy positively that had been regarded as an interference to magnetic refrigeration in the range of intermediate temperatures to room temperature (or a temperature range in which the lattice entropy is large relative to the magnetic entropy) and a method of refrigeration using the entropy was proposed (refer to U.S. Pat. No. 4,332,135). The magnetic refrigeration system is called an AMR system (“Active Magnetic Refrigeration”). A refrigeration device using these systems operates in a strong magnetic field using a superconducting magnet.
A prototype of a magnetic refrigeration device employing an AMR system using an AMR bed filled with fine spherical Gd was made and a continuous steady operation of a magnetic refrigeration cycle in a room temperature region was succeeded. It is reported that realizing refrigeration of about 30° C. by changing a magnetic field from 0 tesla to 5 tesla using a superconducting magnet in the room temperature region is succeeded and that when a refrigerant temperature difference (ΔT) was 13° C., very high refrigeration efficiency (COP=15; however excluding power input to a magnetic field generation mean) was obtained. By the way, refrigeration efficiency (COP) of a home refrigerator and the like in a gas compression/expansion cycle using conventional chlorofluorocarbon is about 1 to 3.
There is an example of a magnetic refrigeration device employing an AMR system using a permanent magnet. This example consists of a turning AMR bed and a fixed magnetic circuit. A magnetic field produces the magnetic circuit in Air gap. It is proved that cooling of 1.5 K could be achieved in a room temperature region using Gd as a magnetic refrigeration material under the conditions of magnetic field: 0.3 T, refrigerant: olive oil, and wheel rotation rate: 4-50 rpm. However, the example is disadvantageous in that a complex structure for accommodating a refrigerant circulating unit in the rotating drive system and an insufficient cooling capability.
Thereafter, a room temperature magnetic refrigeration technology using a permanent magnet is actively developed. A magnetic field application/removal mean in the room temperature magnetic refrigeration technology is roughly classified to a rotating type and a reciprocating type. The rotary type is a system in which a magnet rotates with respect to a fixed magnetic refrigeration material. In contrast, the reciprocating type is a system for relatively reciprocating a magnetic circuit for generating a magnetic field and a magnetic refrigeration material by moving any one of them. A piping system becomes complex in the rotating type, and a magnetic delay occurs in correspondence to a moving stroke when a magnetic field is applied and removed in the reciprocating type.